This invention relates to apparatus for mixing fluids, and in particular to mixing blade constructions for mixing liquids in containers.
Mixing blade constructions in the form of turbines or impellers for mixing liquids and for suspending solids in liquid-solid slurries in tanks and other containers are known and widely used. (The terms mixing blade assembly, turbine and impeller are used interchangeably herein). In the course of the development of such blade constructions, it has been recognized that there are competing factors to be considered in designing mixing blade assemblies and in evaluating their performance. On one hand, there should be a high degree of mixing. This is best achieved by providing for a high degree of axial flow of the fluid in the direction of the axis of rotation of the mixing device. Other less productive flow patterns are radial and tangential or rotary. On the other hand, there should be a low power consumption for achieving the mixing of the fluid. The performance of a mixing blade can be expressed in terms of its pumping efficiency: EQU e=.sup.Q /P
where
e=pumping efficiency
Q=axial fluid flow rate (pumping output)
P=power consumed (power input)
A number of factors affect the axial flow rate. There is ideally purely axial flow as the mixing blades of the mixing blade assembly turn through the fluid. In other words, a force analysis of the respective blades ideally consists of axial force vectors. In actuality, there are rotational or drag forces which resist rotation of the blades as well. Marine turbines having very high axial flow characteristics are known, but in all but very small tanks the size of the blades makes marine turbines inappropriate for mixing applications. Standard axial flow turbines yield a high degree of axial flow, but the presence of significant radial components results in a stagnant region of fluid near the turbine. Impellers for fluid mixing purposes are configured in special ways in efforts to achieve high axial flow. For example, U.S. Pat. No. 4,147,437 proposes a curved plate airfoil impeller to obtain an axial flow pattern. Controlled axial flow over long distances improves the degree of mixing both through greater vertical distance of fluid movement from the impeller, and enhanced secondary mixing flow patterns, and enables the disposition of the mixing blade assembly high in the tank; the high location makes possible a shorter drive shaft, and further prevents the impeller from getting stuck in slurry accumulating near the bottom of the mixing vessel. Likewise, enhanced axial flow can be used to mix liquid at any depth by positioning an impeller near the base of the vessel for pumping liquid upwardly. U.S. Pat. No. 4,468,130 proposes a mixer blade construction which incorporates a geometric pitch angle (the angle between the chord line of the curved blade and a plane perpendicular to the axis of the drive shaft) which increases from the tip of the blade to the base and which is at the threshold for fluid flow separation from the surfaces of the blade so that maximum axial flow occurs before separation begins.
While efforts have been made to achieve high degrees of axial flow, there are other effects which detract from wholly axial flow. The energy expended in the non-axial flow detracts from the efficiency of the mixing blade assembly. Thus, the presence of turbulence on the blade surface adversely affects axial flow and increases rotational drag and the power required. Various efforts have been made to reduce the likelihood of such turbulence as the impeller rotates through the fluid in the mixing vessel. It is desirable to operate at an angle of attack (the angle of incidence of the fluid on the average chord of the blade profile) well below the stall angle of the blade. Although the foregoing U.S. Pat. No. 4,468,130 proposes operating at the threshold for fluid flow separation from the blades, such a design could result in undesired flow separation and turbulence when there are two phase fluid systems involved or when the impeller speed changes. It is recognized that turbulence can be reduced by appropriately rounding the leading edges of the impeller blades. It is known to provide these leading edges with cylindrical and elliptical shapes.
The efficiency of mixing blade assemblies can be increased by reducing the energy or power required to achieve a given axial flow rate. Thus, the drag of the device should be reduced as much as possible. Turbulence on the blade surface increases drag. When provision is made to locate the impeller high in the mixing vessel, the length and diameter of the shaft can be reduced. Shorter shafts can be rotated faster than long shafts since shaft vibration increases with length. Besides, shorter shafts are less expensive. Impeller blades in the form of airfoils can be made lighter than flat blades; thin blades are desirable since they are lighter and can be easier to turn. Likewise, the impeller hub should be as light as possible, yet most hubs are cast pieces and are therefore necessarily massive and heavy.
While a high pumping efficiency is very important, there are other factors which are crucial in the overall evaluation of a mixing blade assembly. The assembly should be durable. Impellers for mixing fluids normally must endure significant loads for long periods of time. Complex force and torsion loads are applied to the impeller, and the rotation of the device can result in vibration-caused stresses. Thus, desirable as thin blades may be for example, the blades must be rugged enough to withstand the loads to which they are subjected. Mixing turbines generally comprise a set of vanes attached to a hub. The juncture of the vane and the hub has to be strong. Weak areas can occur when the blade is curved, such as when there is a tendency of the blade to flatten at the juncture as where a curved blade is bolted to a flat surface of the hub. U.S. Pat. No. 4,468,130 noted previously proposes decreasing the camber (the relative sag, which is ratio of the maximum distance between the chord of a blade and the mean line running through the blade across its width) from the tip of the blade to its root to improve the axial fluid flow; however, such an arrangement sacrifices the strength of the blade near the point of attachment.
Impellers are sometimes used to mix corrosive fluids. It is desirable to minimize the corrosive effect on the impeller. Conventional impellers sometimes have exposed surfaces, as between the hub and the vane, where the corrosive fluid can collect.